1. Field of the Invention
The present invention relates to an optical pickup device for use in an optical record/playback apparatus for recording and playbacking data of video and/or sound signals on an optical recording medium such as an optical disk comprising a recording layer with a track structure, more particularly to its tracking servo control of a light beam spot.
2. Description of the Related Art
An optical pickup device for an optical record/playback apparatus generally comprises a light source emitting a light beam, an objective lens for converging the light beam as a spot on an optical disk or recording medium, a photosensitive detector for producing a signal output corresponding to a data signal recorded on the recording medium on receiving light reflected on the recording medium, and an optical system including a beam-splitter arranged across the light path between the light source and the recording medium for directing the reflected light from the recording medium to the photosensitive detector.
Those components are housed in a pickup enclosure which is movable in relation to the recording medium for the so-called coarse adjustment. For the so-called coarse adjustment of accurate positioning the spot, the components of the optical system are adjusted in relative relationship so that data can be recorded onto and retrieved from a desired recording track of the recording medium.
For realizing a high speed accessing to the target recording track, there is a demand reducing the weight of the optical pickup device. For this purpose, the optical pickup device is divided into a movable unit and a stationary unit, in which the movable unit lighter than the stationary unit is arranged to move in relation to the recording medium for allowing a high speed accessing action. The movable unit carries a minimum number of components including an objective lens and an actuator for moving the objective lens in focusing and tracking directions to direct the light beam towards the desired recording track of the recording medium. The stationary unit incorporates a separate optical system containing a light source, a beam splitter, and a photosensor arranged across the light path. The light beam from the light source ID transmitted in parallel from the stationary unit to the movable unit.
There are known various optical recording systems, for example, a phase transition recording system which utilizes the change between crystalline and amorphous of the recording layer of an optical disk upon application of heat; a magneto-optical recording system which utilizes the change of Kerr or Faraday effect of the ferro- or ferrimagnetic recording layer with vertical domains of an optical disk and reverse magnetization upon application of heat and an external magnetic field; and a dye recording system which utilizes the change of photochromic or reflectance of an organic dye recording layer of an optical disk. In these information recording and reproducing systems, pro-grooves may be formed on the surface of the substrate carrying the recording layer as tracks with a predetermined track pitch. This pro-groove is used for tracking servo control to cause light beam spot to trace the pregroove. Recording of data is preformed on or between the pregrooves.
This tracking servo control system for producing a tracking error signal includes the so-called one-beam system using only one light beam for writing or reading data and performing the tracking or the so-called three-beam system using one recording light beam for writing data and two controlling beams for the tracking servo.
In case that an optical information recording medium comprising a recording layer whose state changes in response to a density of intensity of an irradiated light beam, for example, a magneto-optical recording layer is used in the three-beam tracking servo system, two spots of the controlling light beams expand the inverse magnetic domain produced by the irradiation of recording light beam and the external magnetic field during the recording mode, so that the controlling light beams make a problem for recording unwanted data and disturbing the recording of predetermined data to the recording layer of the optical disk.